This invention relates generally to color electrophotographic printing, and more particularly concerns a technique of reproducing an original document having color highlights therein.
In a typical electrophotographic printing machine, a light image of an original document is projected onto the charged portion of the photoconductive member to record an electrostatic latent image thereon. The latent image is developed with toner particles to form a toner powder image. Thereafter, the toner powder image is transferred to a sheet of support material. Subsequently, the toner powder image is affixed permanently to the sheet of support material forming a permanent copy of the original document.
Multi-color printing repeats the foregoing process a plurality of cycles. For example, U.S. Pat. No. 3,531,195 issued to Tanaka et al. in 1970, discloses a multi-color electrophotographic printing machine. As recited therein, the light image is filtered to record an electrostatic latent image on the photoconductive surface corresponding to a single color of the original document. This latent image is developed with appropriately colored toner particles. The toner powder image is then transferred to the sheet of support material. The foregoing process is repeated for successive differently colored light images, each latent image being developed with differently colored toner particles. In this manner, a multi-layered toner powder image is formed. The toner powder images may be fused individually onto the sheet of support material, or after all have been transferred thereto. However, in this type of process, black is formed by a combination of all of toner particles rather than being an independent color. This problem was at least partially resolved in U.S. Pat. No. 3,869,203 issued to Lehmann in 1975. As described therein, the copying machine may operate in one of two modes. In the black and white copying mode, black toner particles are employed to render the latent image visible. Contrawise, in the color copying mode, color copies are created through the combination of cyan and red toner particles. However, it should be noted that even in this type of printing machine a process black i.e., resulting from equal densities of cyan and red toner particles is formed in the color copying mode. Thus, this type of printing machine may either reproduce an original document all in black, or in different colors wherein the resultant black contained within the original document is formed by the process of combining cyan and red.
Process blacks frequently are expensive to create, as two rather costly color toners are required to create this rather inexpensive color. In addition, process blacks frequently have objectionable haloes around the characters due to the misregistration of successive images. Hence, that electrophotographic printing machines herein before developed were limited in their capabilities, in that color copies could not be readily formed with both black toner particles and colored toner particles being deposited on a common copy sheet.
Accordingly, it is the primary object of the present invention to improve color highlighting by employing both black and colored toner particles on a common copy sheet.